It is often desireable to couple a rotary driven member to a rotary powered member in such a way as to transmit a variable speed from the powered member to the driven member. It is also generally necessary to support the driven member on the powered member for relative rotation by a rolling element bearing. This serves to keep the two members concentric and to reduce friction.
An example is the typical vehicle radiator fan drive. A cooling fan driven in one-to-one relation with the engine speed would waste energy at higher engine speeds, a wasteage that increases with the cube of the engine speed. Therefore, the fan is generally driven by a viscous coupling which slips increasingly with increasing engine speed. An example is the fan drive viscous coupling shown in the U.S. patent to Godfrey U.S. Pat. No. 3,259,221. Typically, such a viscous coupling includes a rotary input member rigidly joined to a powered rotary shaft and separated from an output coupling member by a fluid-filled shear space. The output coupling member mounts a fan and is in turn mounted to the powered rotary shaft by a separate ball bearing. This serves to keep the input coupling member co-axial to the output member while allowing it to rotate relative to the input coupling member. The viscous fluid in the shear space provides a slippage between the input and output members at high engine speeds to save energy. It is also known to provide a coupling directly between a fan and the fan drive shaft, as is shown in the U.S. patent to Beier U.S. Pat. No. 2,492,029. There, a generally torus-shaped pair of rubber elements is clamped between a drive shaft and a fan hub to support the fan hub coaxially with the drive shaft and to transmit driving torque therebetween. The rubber elements act as a slightly yielding cushioning element, but do not function as a variable speed coupling.
Another area where it may be desirable to provide a variable speed coupling between a powered member and a driven member is the cooling fan for a vehicle alternator. There is a low or critical speed range where vehicle forward motion is slow, but electrical output is high. While the fan must turn sufficiently rapidly at or below the critical speed range to cool the alternator, it will turn faster than necessary at higher speed and will waste energy. While the situation is similar to the vehicle radiator fan, the radiator fan drive described above would not fit easily in the small space available between a hub of the fan and the alternator shaft. Another problem, in addition to wasteage of energy, is increased noise level at higher fan speeds. The vibration isolator mentioned above would fit into the small space available and provide some noise reduction, but it would not provide a variable speed coupling to save energy.